We have previously demonstrated that a fertilization membrane forms around a sea urchin egg when it is injected with a soluble spermatazoa fraction isosmotic with seawater. Work is proceeding to purify the sperm extract. In addition, we have found that injection of a very small amount of distilled water will cause the formation of a fertilization membrane, presumably by osmotic lysis of an internal organelle and consequent release of its contents.